1. Field of the Invention
The present invention relates generally to spacecraft attitude controllers, and particularly to an on-board system and method for autonomously controlling the attitude of a spacecraft during during all phases of a sub-transfer orbit.
2. Description of Related Art
Following the separation from a launch vehicle, a spacecraft, such as a satellite, will generally need to ascend to an altitude which will enable a geosynchronous orbit to be maintained. This period of ascent is usually referred to as the transfer or sub-transfer orbit. In order to augment the perigee and apogee altitudes of the spacecraft to such a geosynchronous level, a bi-propellant liquid apogee motor ("LAM") is typically employed to generate the necessary thrust. In this regard, it should be appreciated that the stability and attitude of the spacecraft during the transfer orbit period will have a substantial impact upon the amount of fuel which will be expended by the LAM, as well as that expended by other stabilizing thrusters.
One example of a control system which may be used during the transfer orbit phase is set forth in Rosen U.S. Pat. No. 4,961,551, entitled "Stabilization of a Spinning Spacecraft of Arbitrary Shape", which issued on Oct. 9, 1990. This commonly assigned patent is hereby incorporated by reference. The control system described therein essentially provides for two-axis control. More specifically, error signals are used to fire thruster which are duty cycle modulated to provide the equivalence of proportional control over spacecraft attitude. However, this system lacks on-board attitude determination capability and requires a relatively high spin speed for passive spin stability.
Accordingly, it is a primary objective of the present invention to provide an attitude determination and control system (and method), which has on-board three-axes control and optimizing capability during all phases of transfer orbit.
It is a more specific objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which minimizes the dry weight penalty by not requiring that the spacecraft be balanced.
It is another objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which is operable for out of view LAM and precession maneuvers when the spin rate is very low or zero.
It is an additional objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which provides accurate, active, nutation-free, precession capability to support reorientation maneuvers both before and after LAM maneuvers.
It is a further objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which performs active spin-axis precession during LAM burn to minimize cosine arc losses.
It is yet another objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which has the capability to compensate control system interaction with liquid propellant slosh motion at any fill fraction.
It is yet an additional objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which may also be used for on-board orbit prediction.
It is yet a further objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which has active monitoring of control system components and fault protection capability during active or passive transfer orbit phases.
It is still another objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which enables the elimination of configuration and mass property constraints which could otherwise require a roll-to-pitch inertia ratio of a .sigma.&gt;1.05 (for passive stability).
It is still a further objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which optimizes the functioning of a propellant management device ("PMD") by eliminating gas ingestion with active wobble/nutation control at reduced or no spin speed.
It is still an additional objective of the present invention to provide an on-board three-axes attitude determination and control system (and method) which is also applicable to on-station attitude control.